Sorry to stray off-topic a bit, but as mentioned, pocket sliding doors are used across most of the rail manufacturers in Japan, and probably why Hitachi is sticking to something it's more familiar with door-wise for their products. Very few rollingstock in Japan are ever built with plug sliding doors. Most of the shinkansen models are built with pocket doors, compared to European units which prefer outward sliding plug doors in some cases. One model series, the E3 built by Kawasaki and Tokyu (now J-Trec), are fitted with plug doors that are flush to the body, though they still act like pockets doors as they slide inward.
I feel that in the UK and Europe, manufacturers might prefer outward sliding plug doors to pocket doors when it comes to situations about using wall space since it can change the available space within a train. But since Hitachi is using them, it doesn't seem like an issue on that front. Issues with windows and seat alignments are one thing, but that seems to plague other builds regardless of door type and manufacturers.